A photosensitive lithographic printing plate, which has been widely used in the field of offset printing, generally comprises an anodized aluminum substrate having provided thereon a photopolymerizable photosensitive layer and an oxygen barrier film in this order. The recent development of electronic devices has put so-called desktop publishing to practical use, in which input of data, such as manuscripts and images, editing, and proofreading, are carried out on a computer, and which has made it feasible to edit images at a high speed. As a result, it has been demanded to manage plate making on a computer, and attention has been paid to a laser direct printing plate which can be prepared by directly exposing a photosensitive printing plate by scanning with a visible laser beam without using a negative mask. In particular, it has been keenly demanded to develop a printing plate for exposure to a beam of an argon laser having a wavelength of 488 nm and a YAG laser having a wavelength of 532 nm from the standpoint of a broad exposure range. Photosensitive printing plates suitable for exposure to such a laser beam include lithographic printing plates disclosed in JP-A-60-239736, JP-A-5-78410, JP-A-5-107758, JP-A-6-148885, and JP-A-6-34198 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
These conventional photosensitive lithographic printing plates have high sensitivity but tend to show great variation in thickness of a pattern with a slight change in exposure energy of the visible laser and have narrow margins for exposure temperature. They hardly provide a relief pattern with high reproducibility and are very difficult to handle.
To solve these problems, it has been proposed to hold an exposed printing plate at room temperature for 10 to 30 minutes before development or to heat the photosensitive layer at 50 to 180.degree. C. for 5 seconds to 10 minutes either during or after exposure as disclosed in JP-A-57-119343. However, these methods have difficulty in forming a relief pattern with high precision because of involvement of partial collapse of the pattern.